Rich liquid crystal phase behavior of novel alkyl-tri(ethylene glycol)-glucoside carbohydrate surfactants

Abstract

Carbohydrates are appealing non-ionic surfactant head-groups as they are naturally abundant, generally biocompatible and biodegradable, and readily functionalized. Herein, we explore the phase behavior of seven novel carbohydrate-based surfactants (CBS) containing a tri-ethylene glycol (TEG) linker between a glucose head-group and alkyl tail-group, with linear saturated (C8–18) and cis-unsaturated (C18:1) alkyl chains. At high aqueous concentrations, these glycolipid-like surfactants transition into a variety of lyotropic liquid crystalline phases following an expected concentration phase sequence: hexagonal (H1) → bicontinuous cubic (V1) → lamellar (Lα). Using polarizing light microscopy (PLM), a binary (surfactant–water) phase diagram for each surfactant was constructed across a temperature range (25–80 °C) revealing thermotropic behavior and a broadening of liquid crystal phase regions with increasing alkyl chain length. There was also a significant difference between saturated and unsaturated alkyl chains, due to the cis-unsaturated ‘statistical bend’ lowering the melting point. Small-angle X-ray scattering (SAXS) measurements were performed to characterize the liquid crystal phases, identifying highly-ordered p6m,Ia3d, and Lα crystallographic space-groups with up to 7 resolved Bragg peaks, likely due to the highly anisometric nature of the TEG-linked surfactants. The phases were shown to be more numerous and exhibited greater thermal-stability compared to well-characterized alkyl glucoside surfactants lacking an oligoethylene spacer in the literature. Finally, the characteristic dimensions of each phase were determined to enable visualization of the internal microstructures, providing insight into the impact of molecular shape and the distribution of hydro-philicity/phobicity on the formation and stability of liquid crystalline mesophases.